whatever gmail account you initially set your phone up with will be your Market account. unfortunately, google didn't think to give us any easy way to change it after that.

you could try:
1. removing ALL of your gmail accounts from the phone
2. go into your app settings menu
3. clear all of the market's (and the market updater) data and cache, and maybe all updates
4. reboot
5. add your preferred account
6. open up the market

that should reset the account association, after that, just add back in your other accounts